The present application claims priority to Japanese Application(s) No(s). P2000-360262 filed Nov. 27, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a hologram recording medium for recording an information signal in the form of optical phase information. The present invention also relates to a hologram recording/reconstructing apparatus and method for recording and/or reconstructing an information signal in the form of optical phase information.
2. Description of the Related Art
A conventional hologram recording medium is formed of, e.g., a film or a disk. A reference light and an object light, which are each given as a coherent laser beam, are irradiated to a hologram recording medium, whereupon the hologram recording medium records an information signal, which is to be recorded, as an interference pattern of the reference light and the object light, i.e., as phase information of the reference light and the object light. The object light contains an information signal in the form of optical phase information, and is provided as a laser beam reflected from an object or a laser beam modulated by a spatial modulator. Also, a hologram is reconstructed from a hologram recording medium when it is irradiated by the reference light or a reconstruction light that is phase conjugate to the reference light.
There are many reports related to hologram recording/reconstructing apparatuses using disk-shaped hologram recording mediums. For example, U.S. Pat. No. 5,671,073 proposes a method of multiplex-recording holograms with rotation of a disk-shaped hologram recording medium by using a spherical wave as a reference light (hereinafter referred to as xe2x80x9cshift multiplex recordingxe2x80x9d).
Recently, one example of hologram recording mediums obtained by machining a crystal into a disk-like shape has also been reported (Tao Shiquan et al. xe2x80x9cMulti-track storage of 10,000 holograms in a disk-type photorefractive crystal,xe2x80x9d SPIE Vol. 3864, pp. 270 (1999)).
Another conventional method of recording holograms in a hologram recording medium is one for multiplex-recording holograms based on changes of the incident angle of each of a reference light and/or an object light with respect to the hologram recording medium (hereinafter referred to as xe2x80x9cangle multiplex recordingxe2x80x9d).
In the angle multiplex recording, the incident angle of each of a reference light and/or an object light upon a hologram recording medium must be changed using a beam deflector or the like.
To perform the angle multiplex recording on a hologram recording medium, various methods using a beam deflector are known for changing the incident angles of a reference light and/or an object light.
Methods for changing the incident angle of each of a reference light and/or an object light by a beam deflector include one of mechanically controlling the incident angle by using a galvanometric mirror or the like, and another one of electrically controlling the incident angle by using an acousto-optical deflector (hereinafter referred to as an xe2x80x9cAODxe2x80x9d), an electro-optical deflector (hereinafter referred to as an xe2x80x9cEODxe2x80x9d), or the like.
Assuming here that the aperture width of a beam deflector is D, the aperture shape factor is a (1.22 for a circular shape and 1 for a rectangular shape), the wavelength of a reference light and/or an object light irradiated to a hologram recording medium is xcex, and the angle amplitude of the reference light and/or the object light is xcfx86, the number N of resolving points of a beam deflector is given by the following formula 1:                     N        =                              φ            ⁢                          xe2x80x83                        ⁢            D                                a            ⁢                          xe2x80x83                        ⁢            λ                                              (        1        )            
From the formula 1, it is understood that the number N of resolving points increases as the aperture width D has a larger value and the angle amplitude xcfx86 of the reference light and/or the object light has a larger value. Because the product of incident height and incident angle of the reference light and/or the object light at each plane is constant based on the Lagrange-Helmholtz relationship, the number N of resolving points remains the same even when beam shaping optical systems are disposed before and after the beam deflector.
Still another method for changing the incident angle of each of a reference light and/or an object light irradiated to a hologram recording medium is one of deflecting a beam with a wedge-shaped prism. In other words, it is known that the direction in which a reference light and/or an object light propagates can be changed in small amount and can be adjusted in small angle by rotating a wedge-shaped prism. Accordingly, the reference light and/or the object light can be deflected in any desired direction. Then, by setting an apical angle of the wedge-shaped prism to a smaller value, a deflection angle due to the prism rotation can be reduced. This method is therefore effective in adjusting the direction, in which the reference light and/or the object light propagates, in small angle.
New methods for hologram multiplex recording have also been proposed recently. One example of those methods is the so-called peristrophic multiplex recording described below in detail (Kevin Curtis et al. xe2x80x9cMethod for holographic storage using peristrophic multiplexing,xe2x80x9d 19, Opt. Lett. 993 (1994) and A. Pu et al. xe2x80x9cHigh density holographic storage in thin film,xe2x80x9d SPIE Vol. 2338, Optical Data Storage (1994), 69).
In the peristrophic multiplex recording, as shown in FIG. 33, multiplex recording of holograms is realized by rotating a reference light 41 in the direction of an arrow R2 along a conical surface with its apex defined by a part of a disk-shaped hologram recording medium 40.
Also, in the peristrophic multiplex recording, the degree of multiplexity can be further increased by changing the incident angle of the reference light 41 upon the hologram recording medium 40 in the direction of an arrow R3 so that the above-mentioned angle multiplex recording of holograms is realized in the direction of radius vector in a combined manner.
Assuming here that the Bragg angle is dxcex8, the wavelength is xcex, the thickness of the hologram recording medium 40 is t, the incident angle of the reference light 41 upon the hologram recording medium 40 is xcex8R, and the incident angle of an object light 42 upon the hologram recording medium 40 is xcex8S, the Bragg angle is dxcex8 in the peristrophic multiplex recording is given by the following formula 2:                               d          ⁢                      xe2x80x83                    ⁢          θ                =                                                            2                ⁢                λ                            t                        ⁢                                          cos                ⁢                                  xe2x80x83                                ⁢                                  θ                  s                                                            sin                ⁢                                  xe2x80x83                                ⁢                                                      θ                    R                                    ⁡                                      (                                                                  sin                        ⁢                                                  xe2x80x83                                                ⁢                                                  θ                          R                                                                    +                                              sin                        ⁢                                                  xe2x80x83                                                ⁢                                                  θ                          s                                                                                      )                                                                                                          (        2        )            
Additionally, in usual angle multiplex recording of holograms, the Bragg angle dxcex8xe2x80x2 is given by the following equation 3 wherein the refractive index of the hologram recording medium 40 is n:                               xe2x80x83                ⁢                              d            ⁢                          xe2x80x83                        ⁢                          θ              xe2x80x2                                =                                                    λ                ·                cos                            ⁢                              xe2x80x83                            ⁢                              θ                s                                                                    n                ·                t                ·                sin                            ⁢                              xe2x80x83                            ⁢                              (                                                      θ                    R                                    +                                      θ                    s                                                  )                                                                        (        3        )            
As conventional methods for reconstructing holograms from a hologram recording medium, there is known a phase conjugate reconstructing method which employs, as a reconstruction light, a phase conjugate light having the same phase as a reference light but propagating in an opposite direction to the reference light. A method for generating a phase conjugate light to perform the phase conjugate reconstruction is practiced, for example, by splitting an irradiated reference light with a beam splitter and creating an optical path separate from the reference light irradiated for recording holograms.
When recording and/or reconstructing holograms using the conventional hologram recording mediums described above, however, the number N of resolving points obtained with the methods using an AOD and an EOD as beam deflectors are about 1000 or several tens, respectively. Those methods have limitations in further increasing the degree of multiplexity in excess of the respective number N of resolving points. Another problem is that, to maximize the recording density while minimizing crosstalk noise, the deflection angles of the reference light and/or the object light must be controlled with accuracy on an order of several-thousandths of one degree.
Also, the method of mechanically controlling the incident angle by using a galvanometric mirror or the like, which serves as a beam deflector, has a problem in that a deterioration occurs in reproducibility due to, e.g., a backlash, in resolution accuracy, and in stability against disturbances.
The peristrophic multiplex recording has a problem in that a complicated and relatively large-size device is needed for deflecting the reference light.
The phase-conjugate reconstructing method has problems in that an optical system having an increased size is required to provide a separate optical path for generating the reconstruction light, and a beam deflector for the reference light must be disposed in each optical path when carrying out angle multiplex recording of holograms.
In view of the state of the art set forth above, it is an object of the present invention to control the deflection angle of each of a reference light and an object light incident upon a hologram recording medium with higher accuracy, to improve the degree of multiplexity in recording of holograms, and to increase the recording capacity of the hologram recording medium.
To achieve the above object, the present invention provides a hologram recording medium for recording an information signal in the form of optical phase information upon irradiation of a reference light and an object light while the hologram recording medium is rotated. The hologram recording medium has a first primary surface parallel to a plane in which the hologram recording medium is rotated, and a second primary surface inclined relative to the first primary surface. At least one of the reference light and the object light is irradiated through the second primary surface, and the hologram recording medium has light transparency allowing at least a part of the irradiated reference light and object light to pass therethrough.
With the hologram recording medium having the above features of the present invention, the reference light irradiated to the hologram recording medium is deflected with rotation of the hologram recording medium, whereby an information signal can be recorded as phase information of the reference light and the object light through angle multiplex recording. As a result, the recording density can be improved.
Also, the present invention provides a hologram recording/reconstructing apparatus for recording and/or reconstructing an information signal in the form of optical phase information by irradiating a reference light and an object light to a hologram recording medium. The apparatus comprises a driving system for rotating the hologram recording medium; an optical system having a light source for outputting the reference light and the object light, and irradiating the reference light and the object light to a hologram recording medium having a first primary surface parallel to a plane in which the hologram recording medium is rotated, and a second primary surface inclined relative to the first primary surface, at least one of the reference light and the object light being irradiated through the second primary surface, the hologram recording medium having light transparency allowing at least a part of the irradiated reference light and object light to pass therethrough; and a control unit for controlling the driving system and the optical system.
Further, the present invention provides a hologram recording/reconstructing method for recording and/or reconstructing an information signal in the form of optical phase information by irradiating a reference light and an object light to a hologram recording medium. The method comprising the steps of rotating a hologram recording medium having a first primary surface parallel to a plane in which the hologram recording medium is rotated, and a second primary surface inclined relative to the first primary surface, at least one of the reference light and the object light being irradiated through the second primary surface, the hologram recording medium having light transparency allowing at least a part of the irradiated reference light and object light to pass therethrough; and irradiating the reference light and the object light to the hologram recording medium, thereby recording and/or reconstructing an information signal in the form of optical phase information.
With the hologram recording/reconstructing apparatus and method having the above features of the present invention, by irradiating the reference light to the hologram recording medium of the present invention, an information signal can be recorded and/or reconstructed in the form of optical phase information. Then, when performing angle multiplex recording of information signals in the form of optical phase information, the accuracy in control of an angle, at which the reference light is deflected, can be improved, and therefore the recording density can be increased.